This invention relates to the manufacture of nonwoven fabrics, and more particularly to improvements in the manufacture of a spunbonded nonwoven web formed of substantially continuous filaments.
In the manufacture of nonwoven webs by the well-known xe2x80x9cspunbondxe2x80x9d process, continuous filaments of a molten polymer are extruded from a large number of orifices formed in a spinnerette plate, the filaments are stretched or drawn, and are then randomly deposited upon a collection surface to form a nonwoven web. The stretching or attenuation can be mechanically through the use of draw rolls, or, as is more widely practiced, pneumatically by passing the filaments through a pneumatic attenuator.
Manufacturers of spunbonded nonwoven fabrics have long sought to improve the manufacturing process to achieve higher productivity and better quality and uniformity of the spunbonded nonwoven fabric. Maintaining the quality and uniformity of the fabric becomes a particular concern at higher production speeds and when producing fabrics of low basis weight. Several characteristics affect the quality and uniformity of spunbonded nonwoven fabrics.
Filament separation is the degree of separation of the individual filaments from one another. Good filament separation occurs when the filaments are randomly arranged with limited parallel contact between the filaments. Ideally, no individual filaments should be in parallel contact with another filament, although, in practice, filaments tend to be in parallel contact over considerable distances. Good filament separation is particularly important for lightweight fabrics, where good coverage is more difficult to achieve. Ropiness is the extreme state of poor filament separation. Large numbers of filaments in parallel twisted contact result in long strands in the fabric, which can causes holes or very thin areas in the fabric. Splotchiness is a relative large-scale non-uniformity in basis weight. A fabric having splotchiness is generally weak because of the lower tensile strength of the thin areas of the fabric. Also, a splotchy fabric generally has poor cover properties.
Many attempts have been made to address the problems of poor filament separation, ropiness and splotchiness while still preserving the tensile properties of nonwoven webs made from spunbond thermoplastic filaments. For example, U.S. Pat. Nos. 3,296,678; 3,485,428 and 4,163,305 describe various apparatus and methods for mechanical and pneumatic oscillation of continuous filament bundles to spread the filaments as they are deposited on the collection surface. U.S. Pat. No. 4,334,340 describes using an airfoil at the exit of a round attenuator tube to separate continuous filaments prior to their deposit on a forming wire. Forced air follows the leading edge of the air foil and filaments striking the foil are carried by the forced air onto a forming wire, resulting in a spreading of the filament bundle that promotes random deposit of the filaments.
Various electrostatic methods have been proposed to promote spreading of the filament bundle by applying an electric charge to the filaments to cause the filaments to repel one another. U.S. Pat. Nos. 3,338,992 and 3,296,678 describe electrostatically charging the filament bundle with an ion gun or corona discharge device prior to drawing and forwarding the filaments. U.S. Pat. No. 5,397,413 describes a process for producing spunbond nonwoven fabrics wherein the filaments are attenuated with a slot shaped pneumatic attenuator and wherein the filaments are electrostatically charged to enhance filaments separation.
A number of spunbond manufacturing processes employ a diffusion chamber located between the pneumatic attenuator and the collection surface to assist in controlling the airflow and thereby achieving improved formation. For example, devices of this general type are shown in the apparatuses described in U.S. Pat. Nos. 3,334,161; 4,812,112; 5,211,903; 5,439,364; 5,814,349, and in published applications WO 00/65133 and WO 00/65134.
While the known apparatus and processes are satisfactory in many respects, it is still recognized that the formation of a spunbond fabric is not as uniform and consistent as is desirable, and that the need exists to continue to improve the uniformity of a spunbond nonwoven fabric.
Accordingly, it an object of the present invention to provide improvements in the manufacture of spunbond nonwoven fabrics, and in particular to provide for improved formation of the filaments into a spunbond nonwoven fabric with enhanced uniformity.
In accordance with the present invention, it has been discovered that the aerodynamic behavior of the airflow in a region just above where the filaments are deposited on the collection surface chamber plays an important role on the uniformity of formation of the fabric. In accordance with the invention, a filament diffuser is positioned between the attenuator and the collection surface in the path of filament travel. The diffuser comprises a pair of opposing divergingly arranged side walls and a pair of opposing end walls, these walls collectively defining filament passageway. In accordance with one embodiment of the invention, it has been found that the formation can be significantly improved by injecting a flow of fluid along the walls of the diffuser in the direction of filament travel. More particularly, fluid is injected along both the opposing divergingly arranged walls and the opposing ends walls which form the diffuser.
In another aspect of the present invention, it has been found that the formation can be further improved by electrostatically guiding the filaments. This is achieved by electrostatically charging the filaments and also imparting a like electrical charge to the walls of the diffuser. By independently controlling the electrical potential applied to the respective walls of the diffuser, the path of travel of the filaments through the diffuser can be affected in ways which improve the filament distribution and web formation.
Thus, in accordance with one aspect of the present invention, an apparatus for producing nonwoven fabrics is provided which includes a spinnerette having a plurality of orifices for extruding filaments, an attenuator for receiving and attenuating the filaments, and a collection surface upon which the filaments are deposited to form a nonwoven web. A filament diffuser is positioned between the attenuator and the collection surface in the path of filament travel. The diffuser comprises a pair of opposing divergingly arranged side walls and a pair of opposing end walls, these walls collectively defining filament passageway. At least one fluid injection port is provided in the side walls oriented for injecting a flow of fluid along the side walls in the direction of filament travel. At least one fluid injection port is also provided in the end walls oriented for injecting a flow of fluid along the end walls in the direction of filament travel.
According to another aspect of the invention, an apparatus for producing nonwoven fabrics is provided which includes a spinnerette having a plurality of orifices for extruding filaments, an attenuator for receiving and attenuating the filaments; and a collection surface upon which the filaments are deposited to form a nonwoven web. A filament diffuser is positioned between the attenuator and the collection surface in the path of filament travel. The diffuser comprises a pair of opposing divergingly arranged side walls and a pair of opposing end walls defining a filament passageway. A corona device is provided cooperating with the filaments for imparting an electrical charge on the filaments, and means is provided for imparting a like electrical charge on at least one of the side walls of said filament diffuser so as to thereby guide the filaments as they pass through the diffuser. Preferably, the electrical charge is imparted on at least one of the side walls of filament diffuser by a first power supply electrically connected to one of said the walls and a second power supply electrically connected to the other of said the walls. The first and second power supplies are independently controllable for applying a variable electrical potential to the respective side walls for thereby electrostatically guiding the filaments as they pass through the filament diffuser.